Converters or inverters are commonly used to convert one form of electrical power, such as direct current (DC) power, into another form of electrical power, such as alternating current (AC) power, or vice versa. A multi-level converter is a type of converter that is usually favored over conventional or two-level converters for applications with larger load demands due to the improved power quality, reduced common-mode voltages, higher power densities and lower costs associated therewith. In general, multi-level converters employ switches and corresponding circuitry that couple to a plurality of DC capacitors and/or other DC sources in a coordinated manner that is configured to modulate pulse widths at different DC voltage ranges or levels according to a reference sinusoidal waveform, and ultimately generate the desired AC phase signals.
With the added benefits of multi-level converters, however, also comes increased complexity. More particularly, conventional multi-level converters employ a large number of switches which are switched at high frequencies. For example, a neutral point clamped (NPC) converter implementing a seven-level topology uses a total of thirty-six active switches all operating at high switching frequencies. In another example, U.S. Pat. No. 8,885,374 (“Zhang”) discloses a five-level converter with three DC capacitors and three connected phases, where each phase has one flying capacitor and eight switches, or a total of twenty-four switches. Notably, a seven-level converter based on Zhang's topology would employ as many switches as those in the comparable to seven-level NPC converter above, all actively operating at high frequencies.
Due to the complexity of the circuitry and the number of active switches required, conventional multi-level converters cost more to implement and maintain, and might suffer lowered efficiency due to extra switching losses. Reducing the number of switches and/or reducing the switching frequencies of the switches would be beneficial in several ways. Specifically, reducing the number of switches needed and/or the frequency of switching performed not only reduces the costs of implementing and/or maintaining the multi-level converter, but also generates less heat and thereby prolongs the life of the multi-level converter. Furthermore, reducing the operating temperature of a multi-level converter also reduces the amount of cooling solutions needed in support thereof.
In view of the foregoing disadvantages associated with conventional multi-level converters, a need therefore exists for more cost-effective, simplified and yet reliable solutions that do not adversely affect performance or power output quality. Accordingly, the present disclosure is directed at addressing one or more of the deficiencies and disadvantages set forth above. However, it should be appreciated that the solution of any particular problem is not a limitation on the scope of this disclosure or of the attached claims except to the extent expressly noted.